1. Field of the Invention
The present invention relates to load control devices for controlling the amount of power delivered to an electrical load from a power source. More specifically, the present invention relates to a two-wire dimmer circuit for controlling the intensity of a dimmable screw-in compact fluorescent lamp.
2. Description of the Related Art
A conventional two-wire dimmer circuit 10, as shown in FIG. 1, has two terminals: a “hot” terminal H for connection to an alternating-current (AC) power source 12 (e.g., 120 VAC @ 60 Hz) and a “dimmed hot” terminal DH for connection to a lighting load 14, such as an incandescent lamp. The dimmer circuit 10 typically uses a bidirectional semiconductor switch (not shown), such as, for example, a triac, to control the current delivered to the lighting load 14, and thus to control the state (i.e., on or off) and the intensity of the lighting load between a high-end intensity setting (i.e., a maximum value) and a low-end intensity setting (i.e., a minimum value). The bidirectional semiconductor switch is typically coupled between the hot terminal H and the dimmed hot terminal DH of the dimmer circuit 10, and thus, in series between the AC power source 12 and the lighting load 14. The bidirectional semiconductor switch is controlled to be conductive and non-conductive each half-cycle to control the amount of power delivered to the lighting load 14.
FIG. 2A is a simplified diagram of a hot voltage VH received from the AC power source 12 (as shown by the dotted line) and a dimmed-hot voltage VDH provided to the lighting load 14 when the dimmer circuit 10 is controlling the intensity of the lighting load to the high-end intensity setting. FIG. 2B is a simplified diagram of the hot voltage VH and the dimmed-hot voltage VDH when the dimmer circuit 10 is controlling the intensity of the lighting load 14 to the low-end intensity setting. Using a forward phase control (or “phase-cut”) dimming technique, the dimmer circuit 10 controls the semiconductor switch to be non-conductive at the beginning of each half-cycle of the AC power source 12 during an off time TOFF. Then, the dimmer circuit 10 renders the semiconductor switch conductive during a conductive interval TCON (i.e., an on time) after the off time TOFF. The dimmer circuit 10 maintains the semiconductor switch conductive during the conduction interval TCON until the end of the half-cycle. The intensity of the lighting load 14 is dependent upon how long the semiconductor switch is conductive each half-cycle. At the high-end intensity setting, the off time TOFF is approximately 1.4 msec, such that the conduction interval TCON is approximately 6.9 msec (assuming that each half-cycle is approximately 8.3 msec long on a 120-VAC, 60-Hz AC power source 12). At the low-end intensity setting, the off time TOFF is approximately 6.8 msec, such that the conduction interval TCON is approximately 1.5 msec. Forward phase control dimming is typically used to control incandescent and magnetic low-voltage (MLV) lighting loads.
Gas discharge lamps, such as fluorescent lamps, must be driven by a ballast in order to illuminate properly. FIG. 3 is a simplified block diagram of a lighting system including a fluorescent Tu-Wire® dimmer circuit 20 for driving a two-wire fluorescent load 24. The fluorescent load 24 only requires two connections, i.e., to the dimmed hot terminal DH of the fluorescent Tu-Wire® dimmer circuit 20 and to the neutral of the AC power source 12. The fluorescent load 24 includes a two-wire ballast 26 (e.g., a Tu-Wire® electrical dimming ballast, part number 2W-T418-120-2-S, manufactured by Lutron Electronics Co., Inc., or a Mark X® electrical dimming ballast manufactured by Advance Transformer Co.) and a fluorescent lamp 28. Because of the size of the ballast 26, the ballast is typically located in a junction box external to the lighting fixture of the fluorescent lamp 28. The ballast 26 includes a full-wave rectifier for receiving the dimmed-hot voltage from the dimmer circuit 20, and an active front-end, such as a boost converter, for generating a substantially direct-current (DC) bus voltage. A back-end of the ballast 26 converts the DC bus voltage to a high-frequency AC voltage for driving the fluorescent lamp 28.
The Tu-Wire® dimmer circuit 20 is specifically designed to drive the fluorescent load 24 and may comprise part number NTFTU-5A or part number SFTU-5A3P, both manufactured by Lutron Electronics Co., Inc. The ballast 26 controls the intensity of the lamp 28 in response to the amount of time that the semiconductor switch of the dimmer circuit 20 is conductive each half-cycle. The ballast 26 requires a minimum input voltage greater than the minimum input voltage of an incandescent lamp or an MLV load, so that the low-end intensity setting of the Tu-Wire® dimmer circuit 20 is higher than the low-end intensity setting of the dimmer circuit 10 of FIG. 1 and the lamp does not drop out (i.e., the lamp arc is not extinguished) if the length of the conductive interval TCON is controlled to be too short. Further, because the ballast 26 does not draw as much current as an incandescent lamp or an MLV load, the Tu-Wire® dimmer circuit 20 includes a bidirectional semiconductor switch having a lower holding current rating than the triac of the incandescent dimmer circuit 10 of FIG. 1. Ideally, the triac of the Tu-Wire® dimmer circuit 20 has a holding current rating of approximately 15 mA, where the triac of the incandescent dimmer circuit 10 has a holding current rating of approximately 50 mA.
FIG. 4A is a simplified diagram of the hot voltage VH and the dimmed-hot voltage VDH provided to the fluorescent load 24 when the Tu-Wire® dimmer circuit 20 is controlling the intensity of the fluorescent lamp 28 to the high-end intensity setting. FIG. 4B is a simplified diagram of the hot voltage VH and the dimmed-hot voltage VDH when the Tu-Wire® dimmer circuit 20 is controlling the intensity of the fluorescent lamp 28 to the low-end intensity setting. As shown in FIG. 4A, the high-end intensity setting is the same as the high-end intensity setting of the incandescent dimmer circuit 10 of FIG. 1 (i.e., the off time TOFF is approximately 1.4 msec). Decreasing the high-end intensity setting of the Tu-Wire® dimmer circuit 20 would unnecessarily limit the maximum light output of the fluorescent lamp 28. However, the low-end intensity setting of the Tu-Wire® dimmer circuit 20 is higher than that provided by the dimmer circuit 10 of FIG. 1. Specifically, the Tu-Wire® dimmer circuit 20 provides a maximum off time TOFF of approximately 5.6 msec, such that the semiconductor switch is conductive for approximately 2.75 msec each half-cycle, i.e., at least approximately 33% of each half-cycle. The maximum off time TOFF may range from approximately 5.4 to 5.7 milliseconds (i.e., approximately 31%-35% of each half-cycle) resulting in the dimmed hot voltage VDH having a magnitude of approximately 50 to 58 VRMS when the dimmer circuit 20 is controlling the intensity of the fluorescent lamp 28 to the low-end intensity setting.
Recently, compact fluorescent lamps that comprise screw-in bases for attachment to standard Edison sockets have become popular replacements for standard screw-in incandescent bulbs. These screw-in compact fluorescent lamps consume less power than incandescent bulbs and provide an easy solution for reducing the power consumption of a lighting system. The screw-in compact fluorescent lamps have an integral ballast circuit housed in the base of the lamp and are often made to look similar to incandescent lamps, such as BR30 lamps and PAR38 lamps. Since the screw-in compact fluorescent lamps have different operational characteristics than incandescent lamps, the dimmer circuits used for the screw-in incandescent lamps (as shown in FIG. 1) are not able to appropriately control the screw-in compact fluorescent lamps.
Particularly, problems often arise when the Tu-Wire® dimmer circuit 20 attempts to control the intensity of a dimmable screw-in compact fluorescent lamp to the high-end intensity setting. FIG. 5 is a simplified block diagram of the Tu-Wire® dimmer circuit 20 controlling a dimmable screw-in compact fluorescent lamp 34 (e.g., a Philips® Marathon® dimmable screw-in compact fluorescent lamp), which includes a ballast circuit 36, located in a base portion, and a coil lamp tube 38. FIG. 6 is a simplified diagram of the hot voltage VH and the dimmed-hot voltage VDH provided to the screw-in fluorescent lamp 34 when the Tu-Wire® dimmer circuit 20 is attempting to control the intensity of the fluorescent lamp to the high-end intensity setting. When the dimmer circuit 20 attempts to fire the triac near the beginning of the half-cycle when the hot voltage VH is still relatively small, the screw-in fluorescent lamp 34 may not draw enough current to exceed the latching current rating and/or the holding current rating of the triac in the Tu-Wire® dimmer circuit 20. Therefore, the Tu-Wire® dimmer circuit 20 attempts to fire the semiconductor switch multiple times (as shown by multiple voltage peaks 40 in FIG. 6) until the semiconductor switch is finally rendered conductive. These multiple firings of the semiconductor switch can cause flicker in the light output, audible noise, increased electro-magnetic interference (EMI), and excessive stress on the components of the dimmer circuit 20 and the ballast circuit 36 of the screw-in fluorescent lamp. As a result, the dimming of compact fluorescent lamps has been commercially unsuccessful thus reducing the possibility of further energy savings with these desirable replacements for energy-wasting incandescent lamps.
Therefore, there is a need for a dimmer circuit that provides smooth dimming of a screw-in compact fluorescent lamp and avoids the issues of multiple firings of the semiconductor switch.